ORCID Identifier(s)

0000-0001-5613-6888

Graduation Semester and Year

2017

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Civil Engineering

Department

Civil Engineering

First Advisor

Sahadat Hossain

Abstract

The performance of an evapotranspiration (ET) cover for a landfill exceeds that of a conventional landfill final cover. Its low percolation and high evapotranspiration rely on the properties of unsaturated soils, the energy demands of plants, and the atmosphere. Plant roots pull water out of the cover soil and release it into the environment, thus managing water in a much more natural way than the conventional cover, where water is controlled by creating a physical barrier. Therefore, plant roots are a significant component in the optimization of ET cover performance. In recent years, a great deal of effort has been made to understand the effectiveness of the ET cover system in different regions of the United States. However, comprehensive studies, through field monitoring and model prediction, on the plant root and its effect on the performance of the ET cover are very limited. No research incorporating a thorough study on plant roots has been conducted in the semi-humid region of Texas to evaluate the performance of ET covers. Therefore, the motivation of this study was to develop a methodical approach to investigating below-ground biomass (roots) and to evaluate their effect on the performance of the ET cover. Six instrumented field-scale test sections (Lysimeter) of soil cover (three on the flat section and three on the slope section), made of 3 ft. thick compacted clay overlain by 1 ft. thick topsoil, were constructed at the City of Denton Landfill, TX and monitored for three and one-half years. Three different types of vegetation were planted in the test sections. Eight acrylic plastic tubes (minirhizotron) were installed in the six test sections to determine the root zone depth, root distribution and assess the root dynamics. Root images were captured from minirhizotrons to quantify the roots in terms of length through image analysis. A systematic approach was undertaken for enhancing the image quality before quantification. Traditional root sampling and electrical resistivity imaging on the cover were conducted to verify the results obtained from the minirhizotron. To evaluate the changes in the soil properties, a field soil water characteristic curve (FSWCC) was developed, based on the instrumentation results. A Guelph permeameter was used to determine the time-dependent saturated hydraulic conductivity of the cover soil. Measured root depth and distribution was found to be limited to certain depths. The maximum root depth found was for Bermuda grass (nearly 20 inches). Soil density was found to be a resistive factor for root growth. Field evapotranspiration (ET) from water balance measurements was found short of potential evapotranspiration (PET) due to the lack of adequate root depth. Bermuda grass was found to perform relatively better than other grasses in terms of annual transpiration. No significant difference was observed in the annual percolation (45 mm to 80 mm) of all the lysimeters throughout the monitoring period. The major pulse of percolation occurred during high intensity rainfall. Finally, the water balance of the lysimeters was simulated, using the UNSAT-H code. A forward model with a conservative approach and field-fit simulation were conducted to compare the field water balance. Field-fit simulation yielded results that were close to those of the field-monitored results. A parametric study was conducted to evaluate the climatological parameters and the critical soil and plant parameters. Parametric study revealed that increased root depth is more important than shallow root depth with high density to reduce annual percolation. Annual precipitation with frequent high intensity events causes the major increment in annual percolation. Saturated and unsaturated hydraulic properties also play significant roles in the amount of annual percolation.

Keywords

Water balance

Disciplines

Civil and Environmental Engineering | Civil Engineering | Engineering

Comments

Degree granted by The University of Texas at Arlington

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